In order for a force of gravity to be present there must be a ‘force of attraction’ and some physical ‘movement’ between two objects. To notice any force of gravity an object with a small mass must be pulled towards the center of another object with a far larger mass.
In the image below ‘base’ jumpers are being pulled towards the center of the earth. The center of the earth is located some 3959 miles (6371 km) down below!
We know how the force of gravity affects us all, living as we do, on the surface of planet earth.
But do we know how the force of gravity affects us if we are 250 miles above our planet? We probably know very little since not many of us get the chance to go there!
Before we answer this question we need to understand how the force of gravity makes it difficult for us humans to reach space.
To reach space a rocket has to escape the force of earth’s gravity. Before it can escape the force of earth’s gravity a rocket has to accelerate to very fast speeds.
To attain these incredibly fast speeds a spacecraft has to carry enormous external storage tanks to provide rocket fuel to power very thirsty engines.
A rocket leaving planet earth has to accelerate to a speed of 25, 038mph ( or 40 294 kmh) before it can ‘break free’ from the force of the earth’s gravity, switch off its engines and ‘float’ in orbit around the earth. 25, 038mph is called the ‘escape speed’**
Once empty, these external fuel tanks are no longer of use and are jettisoned into the sea. (pulled down by the force of gravity!)
By the time it reaches a height (or altitude) greater than 62 miles (100kms) above the surface of the earth the space shuttle will be able to switch off its engines and ‘float’ in space. However, it will only be able to ‘float’ in space so long as it is orbiting the earth at a fast enough velocity. (more of which later)
The space shuttle does not now face any immediate risk of being pulled back down to earth by the force of gravity.
The force of gravity is 83% weaker on the moon than it is on planet earth. If a space craft wishes to escape the force of the moon’s gravity so it can orbit the moon without its engines on it would need to accelerate to 8,640 mph. (13905 kmh)
The force of gravity on the surface of Jupiter is 2.5 times greater than the force of gravity on the surface of planet earth. A rocket blasting off from the surface of Jupiter (if that were ever possible!) would have to accelerate to an ‘escape speed’ of 133,097 mph (214, 200 kmh) so it could reach an altitude at which it could orbit Jupiter with its engines switched off.
The force of gravity on the surface of the sun is 28 times greater than on planet earth. Our pretend rocket blasting off from the surface of the sun would have to reach an incredible escape speed of 1,381,308 mph (2,223,000 kmh) before it could reach an altitude at which it could orbit the sun with its engines switched off.
The gravitational force on Mars is half as strong as the gravitational force on the earth. Any rocket leaving Mars would have to reach an escape speed of 11,184 mph (18,000 kmh) before it could either switch its engines off and orbit the planet or carry on flying through space back to planet earth.
Of course there is! But you would not have thought so looking at these ‘weighless’ tomatoes being juggled on the International Space Station!(ISS)
In a low earth orbit, only 250 miles above the earth’s surface, the force of gravity is only 10% weaker than it is on the surface of the earth. If this is the case, then there MUST be a force of gravity pulling the ISS towards the center of the earth.
The answer lies both in the amazingly fast speeds the ISS reaches and the fact that the ISS orbits our spherically shaped planet.
If the earth were flat, the outcome would be entirely different!
1) Velocity of the ISS as it orbits the earth
At a speed of 17,000 mph (27 600 kmh) or just over 4 miles (7.66 km) a second, the ISS travels 10 times faster than a speeding bullet!
The speed of 17,000 mph is critical. If the ISS flies faster than 17,000 mph the force of gravity will be too weak to keep the ISS orbiting the earth and the ISS will fly off into deep space.
If its speed is slower than 17,000 mph, the force of gravity will be too strong and the ISS will come crashing down to earth.
2) The spherical shape of the earth
The spherical shape of the earth is very important in ensuring that the ISS stays in orbit.
The ISS stays orbiting the earth when its speed exactly matches the curvature of the earth.
By the time the force of gravity has pulled the ISS downwards 5 meters towards the center of the earth, the surface of the earth is 5 meters further away!
By the time the force of gravity has pulled the ISS downwards by 1500 meters, the surface of the earth is 1500 meters further away!
By the time the force of gravity has pulled the ISS downwards by 15 000 meters, the surface of the earth is 15 000 meters further away!
By the time….etc!
In effect, the ISS is forever falling down,down, down….. but never gets any closer to the ground. We call this phenomenon ‘freefall’.
Not only is the ISS forever being pulled towards the center of the earth by the force of gravity, but the astronauts on board the ISS are also forever being pulled towards the center of the Earth.
They are being pulled downwards at exactly the same velocity at which the ISS is being pulled downwards- that is they are being pulled downards at a velocity of 10 meters per second, every second.(10 meters per second squared)
At the same time they are being pulled downwards at a velocity of 10 meters per second squared they are traveling in orbit around the earth at a velocity 17 000 mph- the same velocity as the ISS!
The surface of the earth is forever disappearing beneath them.
As a consequence astronauts experience a continuous feeling of ‘weightlessness’ or ‘microgravity’.
** often called the ‘escape velocity’. Put simply, velocity means ‘speed with a direction’. If a car travels at 30 mph it travels at a speed of 30mph. If a car travels east at 30 mph it travels at a velocity of 30 mph.